Continuous drying and crystallization process for polyesters

ABSTRACT

WET AMORPHOUS POLYETHYLENE TEREPHTHALATE FLAKE IS DRIED BY EXPOSING THE FLAKE TO A TEMPERATURE JUST BELOW THE CRYSTALLIZATION TEMPERATURE UNDER HIGH VACUUM UNTIL THE MOISTURE CONTENT OF THE FLAKE IS LESS THAN ABOUT 0.05 PERCENT; THEREAFTER RAISING THE TEMPERATURE TO ABOVE THE CRYSTALLIZATION TEMPERATURE WITH MINIMAL MECHANICAL AGITATION.

United States Patent s 746 ass CONTINUOUS DRYING ANn CRYSTALLIZATIONPROCESS FOR POLYESTERS Eugene E. Berkau, Cincinnati, Ohio, assignor to"Ice It is an object of this invention to provide a method of drying wetpolyester flake under conditions which although not requiringelimination of movement, will minimize the necessity for movement.

It is another object of this invention to provide a Monsanto Company,St. Louis, Mo. 5 No Drawing. Filed Aug. 4, 1971, Sen No. 169,125 methodfor drying polyesterflake without the necessity for Int. CL 008g 17/00322/10 a pro-treatment W1th a swelling agent. CL 7 T V 5 Claims It is yetanother object of this invention to provide a simple 1nexpens1ve methodof continuously drymg poly- 10 ester flake. ABSTRACT OF THE DISCLOSUREBriefly, the objects of this invention are accomplished Wet amorphouspolyethylene terephthalate flake is dried by treating the flake byexposure to temperatures just beby exposing the flake to a temperaturejust below the low the crystallization temperature and at a pressureequal crystallization temperature under high vacuum until the to or lessthan about 10 millimeters mercury (absolute), moisture content of theflake is less than about 0.05 peruntil the water concentration as theflake exits from the cent; thereafter raising the temperature to abovethe crysdrying area is low enough to prevent massive sticking; i.e.tallization temperature with minimal mechanical agitaor until themoisture content is below about 0.05 percent; tion. and then exposingthe flake to a temperature above the crystallization temperature of theflake at the same pres- BACKGROUND OF THE INVENTION sure with minoragitation.

to of polymer and more Partlcularly drymg flakes chips or plied to abatch process, it is preferred to dry and crystalgranule.s of amorphtmsplymer.1c 1.1mm. pollyesters' lize in a continuous process. Just as inprior art processes, i i g polymer? hnezr a? E both drying andcrystallization may be in the presence of t afvmg b q Prope 2 are air oran inert gas. When drying is accomplished before pose o a stanila13.0mm! (a eas 0 y W crystallization, there will be little or nosticking or hy- Of an ester of dlhydnc alcohol and terephthahc acld'drolysis of the flake Sticking is a result of crystallization Thesepolyesters are these i the polyconden.sa' before all surface moisturehas been removed. Because g g g i g f i g f F F 'fK crystallization isexothermic, the temperature of the partiggg ggy ig 0 ye y me erep a e lse cle increases; and if all or nearly all of the surface moisture hasnot been removed, plasticization of the extremities of It 1s often thepractice that polyethylene terephthalate, the particles will occur.having been polymerized, 1s extruded mto rods or ribbons which arequenched by water. The rods are then cut into EXAMPLES flakes orgranules in which form they may be subjected to further Polymerizationin the Sohd State; or y may he Polyethylene terephthalate flake isexposed to a tempermelted and FQ X forced through a spmneretteature of82 for 1.5 hours at a pressure of 10 millimeters In either case, it isessential that the flakes or granules be mercury absolute in a scmwconveyor, precise dw ll ti thqmughly dned' A {lumber of methfids drymgof h 40 being adjusted until water concentration at the end of the t hibr igi' a t l h g s ih nf z frhm2 :wefs thfa highrig drying stage issufficient to prevent massive sticking of the of the flake attemperatures below the melting point while The flake then progressesInto the crystalhzatlon tumbling the flake on a rolling drum Tumblingwas neces ,sectlon where under separate control, the temperature of saryto avoid the sticking of the flakes together which bethls Pol-non ofthehconviyor lshralsed l t comes a serious problem, particularly inacontinuous procremams P P t rolfg out t e extru i ess clogging machineryand causing stoppages It has the crystallization section of the conveyoris not critical. been taught to avoid sticking by treating the amorphouscfryslanlzatwn Qccurs P small afhouht of agita' granules with a lliagent d exposing h to t1on is used to discourage the minor sucking ofthe flake. peratures up to about 110 C. to cause a partial crystal- 5Ten hp h the flake Were exposed the fOhOW- lization before drying at yethigher temperatures. ing conditions in the above descnbed screwconveyor:

TABLE Drying conditions at- Pres- Length Heat temp. 0.) sure DwellThruput Agitator of experi- (mm. time (grams! speed ment Ex. EntranceExit Hg) (hours) 'min.) (r.p.m.) (hours) Comment 150 3. O 2. 0 8. 0 7 2.5 Flow rate fell, flake stuck on agitator. so 150 3.0 2.0 8.0 7 2.5 Do.80 80 3.0 2.0 8.0 7 2.5 Flake temp. lnside=80. 2 3-2 2-2 a a- 2 S5 0 8 20 8 0 7 2 0 Few rate stopped, flake stuck throughout. 95 0.7 2. 0 8. 0 72.0 Flow rate stopped, flake stuck outlet. 85 95 0. 6 2.0 8. 0 7 2.0Flow rate stopped, flake stuck center. 85 0. 5 4. 0 4. 0 4 5.0 Flow ratestopped. 85 90 0.5 4. 0 4. 0 4 3. 5 Flow rate stopped, flake stuck. 8590 0. 5 20 5 Flow rate increased to 43 grams/ in. s5 90 0.5 50 Flow rateimmediately increased to 60 grams/ min.

Examples 1 and 2 were run without agitation. In Examples 3-5, nocrystallization was attempted. Sticking was minimal, and drying wasaccomplished to 0.023, 0.039 and 0.012 percent respectively. As shown inExample 6, temperature as high 95 could not be tolerated in the dryingphase. In Examples 7 and 8, conveyor activity stopped due toinsufiicient removal of moisture in the drying phase. In Examples 9 and10, dwell time was increased, and sutficient water was removed for thecontinuous crystallization process without excess sticking. The flowrate stopped only because of insufficient agitation. In Examples 11 and12, with increased agitation, flow rate was satisfactory.

I claim:

1. A continuous method of drying and crystallizing polymeric flakescomposed of at least 85% by weight of an ester of dihydric alcohol andterephthalic acid comprising:

(a) treating the flake at just below the crystallization temperature ofthe granule and at a pressure equal to or less than mm. Hg absoluteuntil the water concentration is low enough so as to prevent massivesticking of the flake; then (b) exposing the flake to a temperatureabove the crystallization temperature of the flake at the same pressurewith minor agitation.

2. The method of claim 1 wherein treatment of the flake at below thecrystallization temperature continues until the moisure content of theflake is below 0.05%.

3. The method of claim 1 wherein the treatment below the crystallizationtemperature is conducted at a temperature of about C.

4. The method of claim 1 wherein the treatment above crystallizationtemperature is conducted at about C.

5. The method of claim 1 wherein total treatment time is 1-4 hours.

References Cited UNITED STATE$ PATENTS 3,657,191 4/1972 Titzmann et al.260-75 E P 3,014,011 12/ 1961 Zoetbrood 260-75 T FOREIGN PATENTS 635,0701/1962 Canada 260-75 T 558,778 6/1958 Canada 260-75 T 836,742 6/1960Great Britain 260-75 T 850,590 10/ 1960 Great Britain 260-75 T WILLIAMF. ODEA, Primary Examiner W. C. ANDERSON, Assistant Examiner US. Cl.X.R. 34-15

